1. Field of Invention
This invention relates to a process for producing a polyarylene sulfide and, more particularly, to a process for producing a polyarylene sulfide having a high molecular weight.
Polyarylene sulfides (for example, a polyphenylene sulfide) having excellent heat resistance and excellent chemical resistance have attracted great interest in regard to their potential for use in electric and electronic components as well as automotive parts. Being moldable into various formed parts, films, sheets, fibers, etc. by injection, extrusion or other molding processes, polyarylene 5 sulfides are widely used in fields where heat resistance or chemical resistance is needed.
2. Prior Art
Japanese Patent Publication No. 3368/1970 discloses a process for producing a polyarylene sulfide wherein the reaction between a dihalogenated aromatic compound and an alkali metal sulfide such as sodium sulfide or the like is conducted in an aprotic polar solvent such as N-methyl-2-pyrrolidone.
However, the molecular weight of the polymer obtained by this process is not sufficiently high to be used for injection molding or the like. Therefore, this low-molecular weight polymer is heated in the air so as to be crosslinked through oxidation, thereby enabling the polymer to be used in a molding process. However, even such a polymer having an increased molecular weight suffers from inferior extrudability which may be attributed to the high level of crosslinking and branching, so that it has heretofore been difficult to form said polymer into films or fibers.
Under these circumstances, processes for obtaining a high-molecular weight polyarylene sulfide through polymerization reaction have been proposed. A typical example of this type of prior art is a process disclosed in Japanese Patent Publication No. 12240/1977 in which polymerization reaction is conducted in the presence of R-COOM (wherein R is a hydrocarbyl group and M is an alkali metal) which is employed as a polymerization auxiliary. The high-molecular weight polymer obtained in this way is considered to be excellent in terms of extrudability and hence applicable to formation of films, fibers, etc.
The above-described process, however, suffers from the following problems. Namely, the polymerization auxiliary that is used in the process is required in an amount which is substantially equimolecular with that of sodium sulfide, and the lithium acetate which is effective in increasing the molecular weight is costly. Accordingly, the production cost is unfavorably high and the process is therefore disadvantageous from the industrial point of view. The process has the additional problem that, since polymerization is carried out in the presence of a large amount of a water-soluble salt of an organic acid, a large amount of salt of an organic acid is mixed in waste water discharged from the processing step carried out after the polymerization which may give rise to pollution-related problems, and it is costly to remove the contaminant.
U.S. Pat. No. 4,038,263 discloses a process wherein lithium halide is used as a polymerization auxiliary. Employment of lithium halide enables elimination of the pollution problem and the problem concerning waste water treatment. However, since lithium halide is costly, the problem of high production costs still remains unsolved.
Japanese Patent Public Disclosure No. 7332/1986 discloses a process wherein polymerization is carried out in two stages in such a manner that water is added to the reaction product after the first stage of polymerization and the second stage of polymerization is then conducted at a raised temperature. This process enables elimination of the pollution problem and the problem concerning high production costs resulting from the use of a costly polymerization auxiliary. However, examination of this prior art process conducted by the present inventors has revealed that this process lacks effectiveness in increasing the molecular weight and, since the pressure during the second stage of polymerization is considerably high, a high-pressure reaction vessel must be used and the fixed cost is therefore increased.